For a number of reasons it is a desire to know exactly how much metal which at all times is present in a casting furnace before start of casting, during the casting process and after finishing the casting process. During semi-continuous casting of bolts and rolling ingots of aluminium it is used casting furnaces which may contain 60-80 tons of molten aluminium. It is important that one prior to the casting process knows the amount of aluminium in the casting furnace in order to ensure that the bolts or the rolling ingots can be cast to a specified length. Further, it is important to know the amount of aluminium remaining in the furnace after finishing of the casting process, as the remaining amount of metal in the casting furnace will constitute the start of the next batch of aluminium prepared in the furnace, and one has to take this amount of metal into account in order to obtain a correct analysis of the aluminium alloy during production of the next batch in the furnace.
It is further known that the effective volume of casting furnaces changes during use of the furnaces, as the linings in the furnaces are subjected to wear resulting in an increased volume and build up of dross resulting in reduced volume. For instance, new casting furnaces for aluminium may contain 60 tons of molten aluminium while they after two to three years of use may contains 70 tons.
It is known to determine the amount of metal in such furnaces by weighing, but for a number of reasons it has been found that it is difficult and very costly to maintain a stable weighing system for such furnaces. Thus the furnace construction itself may weigh 200-300 tons and is subjected to substantial thermal and mechanical strain during operation. Further it is difficult to include volume changes in the furnace due to wear of lining, as this is dependent on completely emptying the furnace in order to weigh the empty furnace. Taring of a weighing system also necessitates complete emptying of the furnace. Calibration of a weighing system also necessitates complete emptying of the furnace and addition of known weights into the furnace. Both these functions will give interruption of the furnace operation. Further a weighing system can only be used to record the amount of metal in the furnace, and can not be used to record the amount of liquid metal in launders, filters etc. between the outlet of the furnace and the casting moulds. Finally a weighing system cannot itself detect when it is out of calibration. This implies that a weighing system must be calibrated or checked regularly.